Abstract

The stress regime of the shallow crust controls the migration of  fluids and fault propagation, subjects of interest to both industry  and academia. Earthquakes are a manifestation of stress release and can be used to study the stress field. A relatively new technology in  the oil industry is the passive seismic monitoring of microseismic  events; a typical downhole array of sensors will record many events  of magnitudes less than Mb-1 per day. Here we present results from oil reservoirs in the North Sea and Middle East. In the former,  events were recorded using sensors in a single borhole, and located  using P- and S-wave travel time delays and the P-wave particle  motion. A noise-weighted array-based analytic principle-component- analysis method was developed to estimate particle motion. The resulting event locations cluster on two neighboring faults. The  events were also used to measure shear-wave splitting, and hence  infer anisotropy, in the overburden. Detailed modelling was used to  constrain the cause of anisotropy to vertically aligned fractures  superimposed on a more intrinsic anisotropy due to phylosilicate  alignment. An exciting observation is a temporal variation in the  shear-wave splitting, which is attributed to stress transfer between  the two adjacent faults. A dataset from an Omani field was acquired  with 8 sensors in 5 wells thus providing accurate source locations  and an image of detailed spatial variations in anisotropy. We have also been able to put constraints on fracture/crack size using analysis of frequency dependent anisotropy. Cumulatively, these  results demonstrate how seismic anisotropy can be used to infer detailed changes in stress and lithology, valuable pieces of  information in the management of an oil field.